The report was later repudiated by Julian Seymour, "director, Lady Thatcher's private office", who said it was "malicious nonsense and entirely without foundation".
It had been alleged that staff in Lady Thatcher's office had circulated a dossier containing pen portraits of Conservative election candidates, "to help the Defence Secretary canvass support". John Whittingdale, Conservative MP for Colchester South and Maldon, one of Lady Thatcher's former aides and the alleged link between her and the Portillo campaign, said: "This story is a total fabrication and utterly without foundation.
"It is clearly designed to cause damage to Lady Thatcher, Michael Portillo and to the Conservative Party. Lady Thatcher's office denied there was any truth in it yesterday, and I am astonished that the Express should see fit to print it, having been told it was untrue."
It has been known for some years that Lady Thatcher was a great admirer of Mr Portillo, and that he would be her current favourite in any contest for the succession to John Major.
But the current leadership campaign has to stay underground because it depends on an inbuilt assumption - widely shared among some contenders - that the Conservatives are heading for election defeat on 1 May.
As the letter from the former leader's office said yesterday: "Lady Thatcher gives her full support to the Prime Minister and will work for victory by the Conservative Party at the forthcoming general election."
But the fact that the covert campaign is being fought is as evident from the dirty tricks as it is from the jockeying for position by potential contenders.
A report in yesterday's Eastern Daily Press, the regional newspaper that serves Gillian Shephard's Norfolk South-West constituency, repudiated a weekend newspaper report that she had decided not to stand in a leadership contest because she wanted to spend more time with her family.
"Friends of the Norfolk MP yesterday poured scorn on a Westminster rumour that she had ruled herself out of standing for the Conservative leadership," the Eastern Daily Press political editor reported.
Using the Westminster code that is commonly used to disguise the person in question, the report added: "Sources close to the SW Norfolk MP said yesterday that the [weekend] article seemed to have something to do with `furthering the cause' of the 35-year-old Welsh Secretary [William Hague]."
The only contender who has so far stood against John Major for the leadership, John Redwood, yesterday maintained his remorseless pace for the next challenge with a speech in which he washed his hands of the current crisis over the Meat Hygiene Service and the state of the country's abattoirs.
"As a minister," Mr Redwood said, "I unsuccessfully opposed the birth of the national Meat Hygiene Service.
"I thought it better to leave the responsibility with the local authorities.
"They are responsible for food safety outside the abattoir, why not let them remain responsible for the abattoirs as well? Again, the new national service has proved to be both dearer and less effective than advertised at the time."